When comparing ExtJS vs Polymer, the Slant community recommends Polymer for most people. In the question“What are the best client-side JavaScript MV* frameworks?”Polymer is ranked 11th while ExtJS is ranked 13th. The most important reason people chose Polymer is:

Web Components are a collection of specifications released by W3C as a way to reduce the complexity of web apps by creating reusable components. Browser support is currently poor for web components, however Polymer is developed to make web components compatible with modern browsers.

Pros

Pro

Provides built in support for UI widgets

Huge amount of widgets available, also easy to build custom widgets by extending the available components.

Pro

Professional support available

If you buy a license you can get professional support and premium forums. Support has short contact times and solutions are of great value.

Pro

Well documented

ExtJS' documentation is very detailed and helpful. All concepts and parts of the framework are thoroughly explained.

Pro

IDE plugins available

Plugins are available for JetBrains, Eclipse and Visual Studio.

Pro

Easy reusability of code

By using the packages and custom components, code can be reused very simply.

Pro

Forum support

Forum support is available.

Pro

Flat learning curve

Anyone with basic JavaScript knowledge can start using this framework.

Pro

Excellent design tools

Eg: Sencha Architect.

Pro

Developer friendly integrated tools

Sencha Cmd, Sencha Inspector and some IDE plugins etc., are the tools which helps developers to speed up their development.

Pro

Based on web components

Web Components are a collection of specifications released by W3C as a way to reduce the complexity of web apps by creating reusable components. Browser support is currently poor for web components, however Polymer is developed to make web components compatible with modern browsers.

Pro

Various basic components

It provides a base component.

Pro

HTML markup is not string

HTML markup as it can be a non-string.

Pro

CSS is easy to apply

CSS can be applied far more comfortably than React.

Pro

Flex layout components

It provides Flex layout components.

Cons

Con

Large footprint

Con

Expensive licensing

ExtJS is free for use in open source projects, but you have to use the unstable version which is riddled with bugs. Furthermore, to build a project (even if it's open source), you have to buy their proprietary tools.

For commercial projects, it costs $665 for a license.

Con

No real support

You only get a limited premium questions that could be answered via the support and if the issue is too large then you might lose all your credits (it has a credit system for support) in one question.In addition the replies in premium forum are sometimes less thought of than you would get in stackoverflow.

Con

Completely unreliable with regard to licensing and pricing

That ExtJs is expensive is not the problem, but what they did is started completely free, then switched the licensing model when they had profited from the community. Also, they suddenly switched from a single user license to a minimum of 5 users.

Con

Lots of bugs

The community is very small and this is because the licencing is very expensive. Therefore not many people to test the framework and give feedback in order to fix.They try to do everything in house which means that a small company (linkedin shows about 100 employees) tries to imitate something very large.The scaling problems lead to lots of bugs and instability.

Con

No web-workers

ExtJS does not take into account the asynchronous nature of todays web browsers where you can do heavy stuff on the background.

Con

Tooling is poor

They had a GUI architect tool, but that tool never was up to date with the current framework version. Now they even seem to have abandoned the whole architect app. Also, to build a ExtJs app you have to use their tool (and then you have to pay, from that moment ExtJs is no longer GPL). But this tool also has lots of bugs.

Con

Steep learning curve

ExtJs is a very opinionated framework. You have to do it the ExtJs way or it's the highway. So, a lot of time, things in your own opinion doesn't make sense or don't work in the way you expect it.

Con

Quantity yes, quality no

There is a large amount of source code that can be useful and indeed all the samples works flawlessly.When you try to get into more complex scenarios you will find yourself in situations where things just do not work as expected. You will have to do manual work.

Con

Slow performance in mobile

Since most mobile web browsers don't have a very powerful JS engine, Polymer can be a little slow for mobile devices.

Con

Lack of browser support

Due to web components being so new, there is still a lot of issues with browser compatibility. Though Polymer has solved some of these compatibility issues with polyfills, there is still a lot of bugs and unpredictability.

Con

No server-side rendering

Polymer does not support server-side rendering. This results in higher loading times, more HTTP requests and it's not very SEO friendly, since search engines have no way of indexing a page if it's not rendered in the server.